In stationary speed monitoring systems, as a rule two piezoresistive pressure sensors (piezo cables) disposed in succession in the direction of travel are used, which are mounted-directly on the surface of the road. When a vehicle drives over the two piezo cables and trips them, corresponding voltage pulses are generated, which define two time reference values t.sub.1, t.sub.2. From the predetermining spacing s of the sensors and the difference .delta.t=t.sub.2 -t.sub.1, an evaluation unit then ascertains the vehicles speed v=s/.delta.t.
A primary disadvantage of such pressure sensors is that they are very vulnerable to external factors. Especially in winter, when ice- and snow-covered roads are strewn with gravel and salt, they are easily damaged and must then be replaced, at great expense.
Using induction loop sensors instead of the aforementioned piezoresistive pressure sensors would indeed lead to sensor arrays that are largely invulnerable to external factors, because the induction loops can be embedded approximately 5 to 10 cm deep in the pavement, but measuring the vehicle speed with such known sensor arrays is generally too imprecise. This is because the induction loops usually form the inductive portion of the resonant circuit of an LC oscillator, whose capacitor is a component of an evaluation unit that is located at a site separate from the measurement site and is connected to the induction loops over relatively long feeder lines. The vulnerability of this kind of sensor array is substantially impaired, however, by the entry of extraneous signals, for instance from nearby induction loops, and by the ohmic feeder line resistances, but also by the required low-impedance output of the end stage of the loop.